The present invention relates to the field of aerosol generation, and, more specifically, to the generation of monodisperse aerosols, that is, aerosols containing particles whose geometric mean diameters are essentially equal.
High efficiency particulate air filters are widely used for cleaning airstreams of particulate contaminants for the protection of people, equipment and the environment. Before being placed into service, it is vital to test these filter materials for their ability to trap particles of known sizes. Such testing is done with devices known as penetrometers, which use an aerosol to challenge the filter material. Particle penetration through the material is generally measured by particle sensing instruments, such as a forward light-scattering photometer, located downstream from the material. In such testing, it is important that the penetrometer produce a well characterized aerosol and it is preferable that the particles of the aerosol be equal, or substantially equal, in size.
Such aerosols are also important in the testing and calibration of air supply and sampling equipment. For example, both electronic and optical instruments are widely used to measure air pollution and emissions from industrial plants. It is imperative that these instruments be accurate throughout their useful lives.
To accurately calibrate these instruments, aerosols containing particles of known size distributions, or preferably, of substantially equal size, are required. Aerosols of this type, having particles of substantially the same size, are referred to herein as monodisperse aerosols.
Monodisperse aerosol generators also are used as a source for injection of uniform particles into internal combustion engines, and as a source of sample solutions into flame and plasma atomic spectrometers. They are also widely applied with mass spectrometers as a means of introducing a solution.
Numerous attempts have been made in the past to create high airborne concentrations of monodisperse aerosols. None has been particularly successful. A recent attempt by the A. D. Little Company used flash vaporization of a liquid followed by ambient cooling to induce self condensation in an attempt to produce equally sized particles. However, testing revealed that this generator was not capable of producing a monodisperse aerosol.
Another method of producing a monodisperse aerosol is disclosed in U.S. Pat. No. 4,687,929 to Browner, et al. Here, a solvent is entrained in a preferably inert sheath gas, such as argon or helium. The entrained solvent passes into a confined space where desolvation occurs. The diameter of the aerosol particles is controlled by the diameter of the orifice through which the solvent flows. This system is primarily intended to supply small quantities of aerosols to a mass spectrometer or liquid chromatograph.
lt is an object of the present invention to provide apparatus capable of generating high concentrations of a monodisperse aerosol at high flow rates.
It is a further object of the present invention to provide a monodisperse aerosol in which the geometric mean diameter (GMD) of the particles in the aerosol can be easily varied at least within the range of 0.2-0.5 .mu.m, and a GMD of less than 1.3 can be maintained.
It is a still further object of the invention to provide a monodisperse aerosol using various organic liquids having low vapor pressures.
It is a still further object of the invention to provide an aerosol in which the particles have a geometric standard deviation (GSD) of about 1.3 or less.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.